Degenerative changes in the intervertebral disc (IVD) are linked to the pathogenesis of chronic low back and neck pain. In the disc, cells of the nucleus pulposus (NP) and the inner annulus fibrosus function in a unique environmental niche, characterized by a very limited vascular supply that imposes hypoxic constraints on the tissue. Our group has shown that the cells of the NP are adapted to their unique environment by the expression and activity of hypoxia-inducible factor 1 (HIF-1). This critical transcription factor regulates many downstream target genes and is responsive to members of the oxygen-sensing prolyl-4- hydroxylase (PHD). Our group has shown that in the NP, HIF-1 and members of the PHD family form a unique regulatory system: while PHD2 mediates partial degradation of HIF-1a, even under hypoxia, PHD3 promotes HIF-1 activity by acting as a transcriptional coactivator. Based on these findings, I propose to investigate the mechanisms that control HIF- PHD system and if its disruption promotes disc degeneration. First, I will investigate if interaction of HIF-1 and PHD2 is controlled by histone deacetylases (HDACs). I will accomplish this by using specific inhibitors and shRNAs targeting HDACs combined with coimmunoprecipitation, proteomics, and site-directed mutagenesis approaches. Next, to determine how PHD3 controls HIF-1 activity and target gene expression in hypoxia, I propose to identify the cofactors that are recruited to HIF-1 by PHD3. Finally, I will test the hypothesis that compromised function of the HIF-PHD system exacerbates degenerative changes of the aging NP by using PHD3-/- mice, and identifying changes in mRNA expression, tissue structure, and biomechanical function of discs from aged mice. These proposed studies together will test the novel hypothesis that precise regulation and proper function of the HIF-PHD system is crucial for NP tissue health, and disruption of the HIF-PHD system promotes disc degeneration. Results from these studies will lead to identification of new pharmacological targets and therapeutic strategies aimed at maintaining proper NP structure and function, and potentially preventing progression of the disease.